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Computed Tomography

The amount of radiation that people are receiving from medical sources is increasing, and this includes children. It is difficult to show directly that radiation doses from CT lead to cancer. However, good data from other sources of exposure show that there are increased cancers in people who have been exposed to radiation at levels that can be encountered by patients undergoing CT scans. This is particularly important in children, whose tissues are more radiosensitive, who receive a larger effective dose for a given level of radiation, and who have a longer time to develop cancers resulting from radiation exposure. For any one person, the lifetime risk of death from cancer is about 1 in 5. While estimates vary, for a child undergoing a single CT of the abdomen and pelvis increases that risk by 1 in 1,000. The risk is cumulative, however, and each subsequent CT scan will increase the risk accordingly. While for any one individual the increased risk is very small, given the large number of CT scans performed the risk to the population as a whole is much larger.

The population of the United States is second only to Japan in per capita CT exams performed. There are approximately 7 million CT studies performed in children every year in the United States, and the number is increasing approximately 10% per year. CT is widely used among all ages of children, with 33% performed in children under 10 years of age. CT is the largest contributor to medical radiation dose in the United States.

Absolutely. There are many techniques that can be used to dramatically lessen the amount of radiation children are exposed to during CT, while still enabling diagnostic quality images (see also What Can I Do? Section). These include: Scan only the area required. Scanning beyond the body regions where there is clinical concern results in needless exposure. Reduce tube output (kVp and mAS). Exposure parameters should be reduced for the smaller patient size. A number of suggested protocols are available (click here)

Perform single phase studies. Most pediatric conditions are readily diagnosable with single phase CT; more phases unnecessarily increases radiation dose without adding to diagnoses.

Use breast shields for girls undergoing chest CT studies.

Should I not order CT scans for my pediatric patients? CT is an extremely useful imaging modality that can provide valuable and even life-saving medical information, and thus can provide more benefit than harm. Like any test, there should be clear reasons to order a CT scan. For many indications, a test like ultrasound or magnetic resonance imaging may provide the same information without exposing a child to radiation. The American College of Radiology (ACR) has imaging appropriateness criteria for a number of pediatric conditions and discusses the utility of various imaging strategies. Discussing the clinical situation and the medical information desired with the pediatric radiologists providing your imaging services can help determine if an alternative test might be better. If a CT scan is needed, make sure that your imaging facility uses appropriate radiation reduction protocols and techniques, and that those interpreting these pediatric studies are qualified.

Without asking, you won’t know. Some facilities may not alter dose technique for studies on children. This website has published a straightforward method that can be implemented at your site with the help of a medical physicist. It is unique in that it does not depend on the manufacturer, model or age of the scanner. While there may be variability depending upon CT scanner manufacturer and institution, there are also a number of published suggested techniques that facilities can use that provide substantial dose savings. Similarly, most adult protocols call for scanning the same area several times (multiple phases); this is rarely required for pediatric conditions and results in needless additional radiation exposure.

Ask:

if your imaging facility is accredited by the American College of Radiology (ACR)

if the CT technologists are credentialed

if a board certified radiologist or pediatric radiologist will be interpreting the study

Should I talk to parents about the risks involved in getting a CT?

The long-term risks of exposure to medical radiation are small but real. However, the diagnostic value that a CT can provide in the short-term usually far outweighs the long-term risks. Most patients are not informed of any potential risks from radiation prior to the exam

(Lee CI, et al. Diagnostic CT scans: assessment of patient, physician and radiologist awareness of radiation dose and possible risk. Radiology 2004;231:393-398), although some institutions are requiring patient informed consent prior to undergoing CT. While it seems like this would deter patients from getting potentially important exams performed, a recent research study found that parents who were told about the risks and benefits of CT still agreed to go ahead and have the study performed (Larson DB, et al. Informing parents about CT radiation exposure in children: it’s OK to tell them. AJR 2007;189:271-275). In short, you should not hesitate to discuss the potential risks of CT radiation with patients and families.

Many medical physicists are highly experienced with multiple advanced imaging modalities, such as mammography, multi-detector computer tomography (MDCT), interventional fluoroscopy, and other imaging modalities in medicine. But providing medical physics services to a dental office requires attention to a different set of issues. This document reviews the major areas of focus for a medical physics survey of a dental office, and addresses relevant image quality and radiation safety principles.

1. The effective doses are so low, why do we worry about dental offices?

a. Remember that many dental patients are children and some offices image asymptomatic patients. Image Gently is committed to helping professionals provide the necessary imaging at the lowest radiation dose consistent with meeting those clinical imaging objectives. The medical physicist is the ideal professional to guide the dental practitioner to methods of reducing dose while providing essential image quality.

Effective doses are low but skin doses for intraoral radiography are relatively high due to the use of direct x-ray film exposure, i.e., no intensifying screens. A full mouth survey may include up to 20 radiographs, each at approximately 200 mrad each resulting in a skin dose on the order of a CT scan, although to a smaller volume.

2. To refresh your memory, here are the most common types of dental images produced:

a. Bitewing images of the chewing surfaces of the teeth with the patient biting down, and typically show four teeth: two maxillary (upper) and two lower (mandibular). These images can be acquired using either small dental film packets or a reusable digital image receptor. (White, 2009) There are two types of digital receptors—computed radiographic (CR) plates and direct digital radiography (DR).

b. Full Mouth Series consists of 16 to 20 films, taken to show all teeth. The actual number of films required varies slightly depending on practitioner preference and the number and size of the teeth in the patient’s mouth. The same equipment used for bitewings is used for all the images in a full mouth series.

c. Cephalometric images are full size radiographs of the skull, typically 8×10-inch, taken either AP or Lateral. Film images are taken in an 8×10-inch cassette with a phosphorescent screen. Digital images are acquired with a digital image receptor, not unlike those used in general radiography.

d. BPanoramic radiographs use an aperture that allows a thin slit of radiation to expose the patient and image receptor as the x-ray tube and image receptor rotate around the patient, with the image receptor translating at the same time. (White, 2009) Panoramic images are used to show the relationship between all teeth and the relevant bones. These images are acquired with a special sized sheet of dental film (15×30 cm or 5×12 inches), placed in a flexible light tight cassette, designed to provide intimate contact with the phosphorescent screen. Digital dental panoramic units may use a slot digital image receptor.

e. Cone beam CT (CBCT) has become increasingly popular in dental specialty offices and is now appearing in general dentistry practices. CBCT systems appear visually similar to panoramic x-ray units, but instead of a slit aperture, the beam is collimated to the 8×10-inch digital image receptor. The CBCT images provide excellent image quality and are being utilized for an increasing number of diagnostic purposes in dentistry. The effective dose may be an order of magnitude higher than from a bitewing, primarily because more organs are exposed. Combined panoramic-CBCT systems have been introduced that utilize a slot shaped image receptors for panoramic imaging and a 2D digital image receptor for CBCT. Some systems have become available with multiple-use selectable collimation that allow the user to reduce the field to the area of interest, but with an associated potential for operator error.

a. If the facility is using dental film, check to see that they are using E-speed film, F speed film , or what is referred to as “E- F speed” film. This may require some detective work, if the film is purchased by mail in bulk. While many facilities have converted to digital image receptors, recent data shows that 31% to 64% continue to use D speed film. (Gray, personal communication; also Table 1) The image quality for E-, F-, and E-F-speed films is similar to that obtained with D-speed film (Bernstein, 2003; FDA, 2014; Ludlow, 2001; Syriopoulos, 2001) at approximately one-half of the radiation dose. (Table 1)

This table also provides some other insights. The average exposure for D-speed film is about 260 mR. This is much higher the necessary. D-speed film exposures should be on the order of 175 to 225 mR.

(Table 2) In other words, there is no need for dental exposures in excess of 225 mR for D-speed film. For histograms showing the wide range of bitewing exposures in use today see Pages 5 and 6 of this PowerPoint presentation.

Table 2 provides suggested exposure ranges for dental bitewing x-ray exposures. As can be seen, the average exposures in Table 1 typically exceeds the suggest exposure ranges in Table 2. For film imaging, this is usually caused by under-processing film due to depleted chemicals. More information can be found here.

Based on 25th and 75th percentile of optimal exposure from Table II from Udupa, H., et al., Oral Surg Oral Med Oral Pathol Oral Radiol 2013;116:774-783. Note—Required exposure for optimal image quality varies with digitalsensor type. Table 2 should be considered as a starting point for image quality and dose optimization.

b. Using the posted technique chart, measure and evaluate the dose. Compare with references. (NCRP, 2003; NCRP 2012) Ask the staff open ended questions like “What techniques do you use for bitewings? Do you have different techniques for children?” If you choose to make alternate technique recommendations, provide a written technique chart for each unit, for adult and pediatric patients. For facilities with multiple x-ray units of different manufacturers or models, it may be beneficial to provide an additional single technique chart showing the differences for all x-ray systems in the suite.

c. Digital image receptors require less radiation skin dose than film imaging. CR (photostimulable phosphor) receptors require a dose similar to E-, F-, or E-F-speed film. DR image receptors can provide clinical images in the 40 to 100 mR range, dependent on the type of digital receptors. Image and radiation dose optimization should be carried out using the data in Table 2 as a starting point. (It is not uncommon to find significantly higher doses being delivered for digital imaging.)

d. Digital imaging has eliminated the need for film processing and all of the associated issues. However, there are issues associated with digital imaging which should be investigated during an evaluation. The medical physicist’s experience with digital imaging in general radiography, dose creep, and challenges associated with the conversion to digital imaging will be fully applicable here. For more information click here.

4. Radiographic film processing conditions and quality vary considerably in the dental imaging community. Verify that proper time-temperature developing is being used. Film processing tips are included in Link B. Film processing is usually the weakest link in the imaging chain, with under processing resulting in low contrast radiographs and increased patient doses. The histograms on Pages 5 and 6 in this PowerPoint presentation clearly demonstrate the broad range of patient doses and suboptimal photographic processing (approximately one-third of the facilities produce films with inferior contrast) in use today. To assist with testing processing conditions, inexpensive devices are available for dental film processing quality control. These are also helpful in assuring appropriate initial film exposure technique selection and processing conditions.

5. Evaluate the x-ray generator and output using standard techniques. Be sure that your radiation detector is properly positioned, and sized appropriately to include the complete x-ray beam. Modern units typically operate at only a single kVp and mA.

6. Carefully evaluate beam collimation. For intraoral radiographic units, consider recommending a rectangular aperture, which has been shown to significantly reduce effective dose by providing an x-ray beam that more closely approximates the rectangular image receptor. (White and Pharoah) The diagnostic medical physicist’s experience with the benefits (image quality and radiation safety) derived from collimating the x-ray beam to the image receptor in body radiography and fluoroscopy (for example), will be directly applicable here. Evaluating collimation for panoramic or CBCT systems requires some pre-planning, and may be accomplished using GAF Chromic Film and the manufacturer’s specifications for geometry.

The photo on the left shows self-developing x-ray film taped to the surface of the x-ray tube cover. The mages at the right show two strips, following exposure.

7. Radiation Safety should be evaluated based on integrated exposure measurements using a dosimeter, taken at various locations within and near the x-ray source. Inquire about workload with the staff, and verify by reviewing representative patient logs. These should be readily available on digital systems. Be sure to account for any effect of very short radiographic exposure times. It may be necessary to increase the mAs to obtain a reading on your survey instrument.

Be aware that CBCT systems are often located where panoramic units were previously installed. However, the scattered radiation dose from CBCT, maybe substantially higher than for panoramic units (about an order of magnitude), due to the significantly larger field of view exposed in CBCT systems. Recent advances of low-dose CBCT options exposes the patients to less or equivalent radiation to commonly used dental imaging modalities (Ludlow 2013). Calculate the exposure to persons in the vicinity of the x-ray source following the principles of NCRP Report 147 (NCRP, 2004). While exposures and workloads may remain consistent from year to year for intraoral x-ray units, increased utilization of CBCT systems in recent years make it essential for the medical physicist to evaluate personnel exposure during each annual survey. At the very least the medical physicist should recalculate estimated weekly exposures using initial area survey measurements and recent workload data. Results should be compared with requirements from the local jurisdiction. Pay attention to personal who work in the line of sight of CBCT units in spite of low doses ,especially with older and high dose units to assure appropriate monitoring and make recommendations to assure.

8. Occupational Dosimetry. While many dental offices are not required to use personnel dosimetry, potentially increased utilization in a digital and CBCT imaging environment may warrant a renewed assessment of occupational exposure with personnel dosimetry. The associated exposure reports are useful to many facilities as a risk mitigation method where many personnel are young females of child-bearing age.

9. Hand-Held Dental X-Ray Units are being used in many offices. One x-ray unit can be used in more than one room. These units must be appropriately shielded by the manufacturer at the x-ray tube and with a leaded-acrylic disk at the front of the position indicating device (PID) to reduce backscatter. Several papers have been published on the safety and image quality of these systems. (Brooks, 2009; Gray, 2012; Phillips, 2012; Thatcher, 2013)

10. Remember that a medical physicist can contribute to the quality of care delivered by a dental facility and personnel safety by following established imaging physics principles for both traditional and newer digital imaging technology. By speaking with the dentist and staff and understanding how x-rays are used in each practice setting the medical physicist’s recommendations can impact a very large number of patients.

Check out these dental imaging tip sheets for further helpful information—

NCRP (2003). National Council on Radiation Protection and Measurements. Radiation Protection in Dentistry, NCRP Report No. 145. National Council on Radiation Protection and Measurements, Bethesda, Maryland.

NCRP (2004). National Council on Radiation Protection and Measurements. Structuralshielding design for medical x-ray imaging facilities. NCRP Report No. 147. National Council on Radiation Protection and Measurements, Bethesda, Maryland.

NCRP (2012). National Council on Radiation Protection and Measurements. Reference levels and achievable doses in medical and dental imaging: recommendations for the united states, NCRP Report No. 172. National Council on Radiation Protection and Measurements, Bethesda, Maryland.

Acknowledgements We would to thank Alan Lurie, D.D.S., Ph.D. for the dental x-ray images.

Background

Medical sources of radiation to the population are increasing. This is of particular concern in children whose tissues are more radiosensitive, whose organs receive a larger effective dose for a given level of radiation, and who have increased time to develop cancers as a result of radiation exposure. It is difficult to demonstrate that radiation doses from medical imaging lead directly to cancer or to state with certainty the exact risk of cancer related to medical radiation; however the committee on the Biological Effects of Ionizing Radiation (BEIR) VII states “… the risk of cancer proceeds in a linear fashion at lower doses without a threshold and … the smallest dose has the potential to cause a small increase in risk to humans.” Good data from other sources of exposure do show that there is an increased incidence of cancers in people exposed to levels now encountered through medical sources. For any one person, the overall baseline risk of death from cancer is about 1 in 5. While estimates regarding additional risk from radiation exposure vary, a child undergoing a single CT of the abdomen and pelvis may increase that risk by 1 in 1,000. While for any one individual the potentially increased risk is very small, the risk to the population as a whole is larger. We do know that the risk is cumulative with repeated radiation exposure. Therefore all studies that expose a child to ionizing radiation should be carefully evaluated as to the potential risk versus the likely benefit.

Fluoroscopic studies entail radiation exposure to not only the patient but also (to a much lesser extent) the fluroscopist and other assisting .personnel. This is especially true in children in whom several persons are often required to effectively immobilize and position the child .

The amount of radiation resulting from fluoroscopic procedures is highly variable, dependent upon fluoroscopic parameters which in turn depend upon several factors, including patient size and desired image detail. The type of procedure performed has a great impact upon patient dose, with those procedures which require long fluoroscopy times, such as interventional procedures, providing the largest doses.

Effective radiation dose for a VCUG has been reported as approximately .5 to 3.2 mSv and an upper gastrointestinal series as 1.2 – 6.5mSv. as compared to the average dose of an abdominal CT in a child of approximately 6mSv.

The availability of endoscopy and CT has resulted in a decline in fluoroscopic procedures; nonetheless fluoroscopy remains an important and frequently used procedure in the pediatric patient, particularly in the US. In a survey of pediatric radiologists conducted by SCORCH (Society of Chairmen of Radiology in Children’s Hospitals) in 2007 the mean number of annual fluoroscopies reported per surveyed hospital was 4,296. Approximately 35% of pediatric fluoroscopic studies are voiding cystourethrograms (VCUG), 30% upper gastrointestinal studies( UGI), and 7% contrast enemas, with miscellaneous categories comprising the remainder. In addition to Diagnostic Imaging, there are several other sources of fluoroscopic radiation, including fluoroscopy for orthopedic procedures, fluoroscopy in the OR for central line placements and other procedures, and fluoroscopy by other services, such as Gastroenterology and Cardiology.

Absolutely. There are many techniques that can be used that dramatically decrease the amount of radiation children are exposed to while still allowing diagnostic quality images (See sections on individual studies (UGI, VCUG, Contrast enema, Information for Radiologists, General Principles for Clinical Diagnostic examinations)

These include: Having a clear understanding of the patient’s problems and goals of the study is important, and providing a detailed history to the radiologist is very important to achieve this goal. It is also very important to limit fluoroscopic time in general and use of magnification mode in particular; careful collimation to the area of interest and appropriate shielding, bringing the x-ray source as close as possible to the image intensifier, matching tube output (kVp and mAs) to the size of the child, utilizing grid- controlled pulsed digital fluoroscopic equipment with adjustable frame speeds and half dose key as well as last image hold and capture capability are also important measures to optimize the examination.

While there may be intrinsic variability dependent upon equipment manufacturer and institution, there are a number of published suggested techniques that facilities can use that provide substantial dose savings. Work with a physicist is very important in the appropriate set-up of fluoroscopic equipment, particularly its adaptation to optimal pediatric imaging.

Fluoroscopic studies are often very useful and can provide valuable and even lifesaving medical information. As with any test, there should be clear reasons to order the study. In some situations ultrasound or occasionally magnetic resonance imaging could provide similar information without exposing a child to radiation. The American College of Radiology (ACR) publishes appropriateness criteria for pediatric conditions that discuss the utility of various imaging strategies. Discussing the clinical situation and medical information with the pediatric radiologists providing your imaging services can help determine whether an alternative test might be better. If a fluoroscopic study is needed, ensure that your imaging facility uses appropriate fluoroscopic equipment, protocols and techniques for children, and that those performing and interpreting these pediatric studies are qualified and experienced.

The diagnostic benefit that the study can provide in the short-term usually outweighs the long-term risks. While it seems that informing parents might deter them from acquiescing with potentially important studies, recent research regarding CT found that parents who were told about the risks and benefits still agreed to have the study performed ( Larson DB, et al. Informing parents about CT radiation exposure in children: it’s OK to tell them. AJR 2007;189:271-275).. You should not hesitate to discuss the potential radiation risks with patients and families.

Without asking, you won’t know. Some facilities may not have fluoroscopic equipment suitable for children or experienced personnel who frequently perform pediatric procedures and may not adequately adjust dose techniques for children or limit the fluoroscopic time and number of spot films or overhead X-rays that are typically obtained in adults.

Ask:

If the facility is accredited by the ACR

If the technologists are credentialed

How frequently the facility performs the requested fluoroscopic study in children

If the equipment is optimized for pediatric patients

If a board certified radiologist or pediatric radiologist will be performing and interpreting the study